Editing Chemical ionization (section)

===Negative chemical ionization===
Chemical ionization for gas phase analysis is either positive or negative.<ref name="pmid7025931">{{cite journal |author=Dougherty R.C. |title=Negative chemical ionization mass spectrometry: applications in environmental analytical chemistry |journal=Biomed. Mass Spectrom. |volume=8 |issue=7 |pages=283–292 |year=1981 |pmid=7025931 |doi=10.1002/bms.1200080702}}</ref> Almost all neutral analytes can form positive ions through the reactions described above.

In order to see a response by negative chemical ionization (NCI, also NICI), the analyte must be capable of producing a negative ion (stabilize a negative charge) for example by [[electron capture ionization]]. Because not all analytes can do this, using NCI provides a certain degree of selectivity that is not available with other, more universal ionization techniques (EI, PCI). NCI can be used for the analysis of compounds containing acidic groups or electronegative elements (especially halogens).<ref name="MSPA"/>{{rp|23}}Moreover, negative chemical ionization is more selective and demonstrates a higher sensitivity toward oxidizing agents and alkylating agents.<ref name=":4">{{cite journal|last1=Dougherty|first1=Ralph C.|title=Negative chemical ionization mass spectrometry|journal=Analytical Chemistry|volume=53|issue=4|pages=625–636|doi=10.1021/ac00227a003|language=en|year=2002}}</ref>

Because of the high electronegativity of [[halogen]] atoms, NCI is a common choice for their analysis.  This includes many groups of compounds, such as [[polychlorinated biphenyls|PCBs]],<ref name=":4" /> [[pesticides]], and [[fire retardant]]s.<ref name=":4" /> Most of these compounds are environmental contaminants, thus much of the NCI analysis that takes place is done under the auspices of environmental analysis. In cases where very low limits of detection are needed, environmental toxic substances such as halogenated species, oxidizing and alkylating agents<ref name="pmid7025931" /> are frequently analyzed using an [[electron capture detector]] coupled to a [[gas chromatograph]].

Negative ions are formed by resonance capture of a near-thermal energy electron, dissociative capture of a low energy electron and via ion-molecular interactions such as proton transfer, charge transfer and hydride transfer.<ref name="pmid7025931" /> Compared to the other methods involving negative ion techniques, NCI is quite advantageous, as the reactivity of anions can be monitored in the absence of a solvent. Electron affinities and energies of low-lying valencies can be determined by this technique as well.<ref name="pmid7025931" />